Variable Frequency Drive System Apparatus and Method for Reduced Ground Leakage Current and Transistor Protection

ABSTRACT

As applications of variable frequency drives (VFD) ( 50 ) continue to grow so do challenges to provide VFD ( 50 ) systems meeting application specific requirements. For multiple reasons to include safety standards and electromagnetic interference, reduced ground leakage current is desirable. Building high output voltage VFDs ( 50 ) using transistors rated at voltages lower than the VFD output voltage is desirable for economic reasons. The apparatus and method described herein meet these challenges and others, in part by placing an electrically insulating plate (cp 176 ) having high thermal conductivity, a low dielectric constant, and high dielectric strength between the heat sink plate of a VFD power semiconductor module and a grounded cooling plate ( 80  TE). The positive effects of this plate installation include reducing ground leakage current induced by system capacitances to ground upon high frequency voltage steps and increasing the effective dielectric strength of the VFD&#39;s ( 50 ) transistor modules engaging in high reliable VFD ( 50 ) voltage output for a given transistor rating.

BACKGROUND OF THE INVENTION

Conventional sinusoidal AC voltage supplies provide only fixed motorspeed and are unable to respond quickly to changing load conditions.With the advent of variable frequency drives (VFDs), a better performingmotor at lower energy costs can be achieved. VFD driven motors rapidlyrespond to changing load conditions, for example in response to shockloads. VFD driven motors provide precision torque output and continuousspeed control, as well. Because of their many advantages, theutilization of VFDs in industrial applications continues to grow.

A conventional medium voltage VFD driven motor system is described belowwith reference to FIG. 1. The neutral point N 26 of the DC bus 20 isgrounded to protect the transistor switches from potential voltagespikes that would cause insulation degradation and component failure.The heat sink plate of transistors in the inverter bridge is alsogrounded, however, the ground connection is not shown in FIG. 1. FIG. 2Aillustrates grounding of the inverter bridge transistor module via theheat sink plate 126 and is described below in greater detail. Againreferencing FIG. 1, three phase cables 30 are connected at one end tothe output terminals 52 of the VFD 50. Cables 30 have an inherentcapacitance per unit length. The total cable capacitance is shown asC_(C) 32. These cables feed the motor M 40, which also has capacitancedue to windings, shown as C_(M) 42, and motor impedance shown as Z_(M)44.

FIG. 3A is a schematic circuit 300 representation of a conventional VFDdriven motor system, for example the drive system shown in FIG. 1.Switch S 60 represents the voltage transitions output from the VFD 50.Upon closing of switch S 60, a voltage transition, from grounded neutralto the positive 22 or negative 23 potential on the DC bus 20, outputfrom the VFD 50 is imposed upon the circuit 300. Ground leakage currentI_(GND) 200 flows freely in the ground connection with the voltagetransitions due to the motor capacitance C_(M) 42 and the cablecapacitance C_(C) 32. Inverter bridge capacitance, C_(IB) 62, isconnected from the neutral point N 26 to equipment ground PE 70 and totrue earth ground TE 80 in parallel with the short circuit of theneutral point N 26 connection to true earth ground TE 80. In thisconventional configuration because C_(IB) 62 is in parallel with theshort circuit connection to ground, C_(IB) 62 contributes negligibly toground leakage current.

Because of their high performance and lower power consumption, VFDs aredesirable in a variety of demanding applications, to include fan andpump loads. However, use of VFDs in medium voltage applications can becomplicated if low ground leakage current is necessary. Low groundleakage current can be necessary in potentially explosive environmentsor in environments requiring reduced electromagnetic interference (EMI).High frequency ground leakage currents, up to the MHz range can lead toEMI, for example in radio receivers, computers, bar code systems, andvision systems.

One example of an application requiring low ground leakage current isunderground mining; the underground mining environment has uniquerequirements and safety standards. Underground mining motors arepreferably in the medium voltage range (between 690 V and 15 kV) and aretypically driven at 4,160 V. A conventional medium voltage VFD providinga 4,160 V output can yield a ground leakage current I_(GND) 200 inexcess of ten amps, which flows from the VFD 50 to the motor M 40 in thegrounding wire. While using a medium voltage motor facilitates the useof smaller cables, the maximum permitted drive to motor ground wireleakage current I_(GND) 200 can be below 1 Amp.

Unlike conventional AC sinusoidal motor drives, VFDs output voltagetransitions on the time order of microseconds. Consequently, largeground leakage currents are induced due to capacitances CM and Cc,inherent in a VFD driven motor system, even at relatively low voltages,for example 690 volts. Referring to FIG. 3A, disconnecting the neutralpoint N 26 of the DC bus 20 from TE ground 80 appears to be a viablemeans of reducing ground leakage current. A schematic representation ofdisconnecting the neutral point N 26 from ground in a conventional VFDsystem 302 is shown in FIG. 3B. As shown in FIG. 3B, disconnecting theneutral point N 26 of the DC bus from TE ground 80 changes the circuitmodel for ground current leakage current, I_(GND) 202. Inverter bridgecapacitance, C_(IB) 62, is now in series with the parallel combinationof cable capacitance C_(C) 32 and motor capacitance C_(M) 42. Thisresults in higher impedance for the ground leakage current due to thedecrease in total system capacitance. However, disconnection of theneutral point N 26 from TE ground 80 leaves transistors S₁-S₁₂ in theinverter bridge susceptible to voltage spikes.

Disconnecting the neutral point N 26 of the DC bus from TE ground 80,leaves the transistors floating relative to the neutral point N 26 ofthe DC bus. Voltage spikes at full DC bus potential can be appliedacross the transistors in the inverter bridge. Referring to FIG. 2A,these voltage spikes are transmitted between the transistors'semiconductor substrate 122 and the transistors' heat sink plate 126across thin insulator 124.

For lower drive voltages, available transistors rated above thedifference between the positive and negative DC bus can be employed in aVFD system having the neutral point of the DC bus disconnected from TEground and left floating. This configuration is successfully employedfor example in SMC's Microdrive 2,300 V model¹. However, when higher VFDvoltage output is needed or desired and when transistors rated at thefull DC bus potential are not practical, protecting transistors fromfull DC bus potential spikes is necessary to prevent reduced componentlife and component failure. Multiple challenges exist for VFD driveapplications. One challenge, for example, is to reduce leakage groundcurrent while protecting the VFD, in particular the inverter bridge.Another challenge is to reduce ground leakage current as much aspossible. ¹ VFD, Microdrive, 2,300V model, SMC Electrical Products,2003.

For other applications, the challenge is to provide a reliable VFDsystem for motors rated at greater than 4160 V. For, example for a motorrated at greater than 4160 V, a VFD providing an output of 6.9 kV outputis desirable. However, presently available transistors to build a VFDwith a 6.9 kV output are susceptible to compromised transistorinsulation and impending component failure. A DC bus rated at 11.5 kV isneeded to achieve a VFD output of 6.9 kV. Inverter bridge transistorsare available at an insulation rating of 5,100 V. Even when the neutralpoint of the DC bus is grounded, the transistor module insulation 124(FIG. 2A) breakdown voltage (5,100 V) is less than half the potential onthe DC bus (11.5 kV). Yet another challenge in VFD systems is to protectthe inverter bridge comprising available transistors connected in seriesto provide VFD output voltages greater than 4160 V when transistors arerated at less than half of the DC bus voltage.

SUMMARY OF THE INVENTION

The present invention provides decreased ground leakage current in a VFDdriven motor system, while protecting the inverter bridge.

It is an object of the present invention to reduce ground leakagecurrent in a VFD driven motor system.

It is another object of the present invention to reduce the groundleakage current by floating the neutral point of the DC bus.

It is another object of the present invention to float the neutral pointof the DC bus while protecting the VFD from component failure due tovoltage spikes.

It is another object of the present invention to increase the impedancefor ground leakage currents.

It is another object of the present invention to further reduce groundleakage current in a medium voltage VFD driven motor system withoutdecreasing system capacitance to ground.

It is another object of the present invention to decrease the totalcapacitance to ground of a VFD motor driven system.

It is another object of the present invention to decrease the totalcapacitance to ground of a VFD motor driven system by means of a highdielectric strength and low dielectric constant plate disposed betweenthe VFD transistor module heat sink plate and a grounded cooling plate.

Another object of the present invention is to improve inverter bridgereliability and component life in a VFD system having transistors ratedat less than half of the full DC bus potential by means of additionaleffective insulation when the neutral point of the DC bus in the VFD isgrounded.

Exemplary embodiments of the present invention can be used in low,medium, and high voltage drive applications.

In accordance with the objects of the present invention, in an apparatusaccording to an exemplary embodiment of the present invention, theneutral point of the DC bus is floating, disconnected from ground.

In an apparatus according to another exemplary embodiment, anelectrically insulating plate having high thermal conductivity, highdielectric strength, and a low dielectric constant is thermally andelectrically connected between the transistor semiconductor substrateand the cooling plate.

In an apparatus according to another exemplary embodiment, a common modefilter is installed at the output of the VFD.

A method in accordance with an embodiment of the present inventioncomprises floating the neutral point of the DC bus and increasing theimpedance of the ground leakage path by means of a dielectric substrate.

Another method in accordance with an embodiment of the present inventioncomprises increasing the impedance of the ground leakage path by meansof a dielectric substrate and also increasing the dielectric strength ofthe transistor module of the VFD to greater than the full DC busvoltage.

Another method in accordance with another embodiment of the presentinvention comprises floating the neutral point of the DC bus, increasingthe impedance of the ground leakage path by means of a dielectricsubstrate, and installing a common mode filter across the three phasedrive cables in a VFD system.

Other objects and advantages of the present invention will becomeapparent to one skilled in the art from the following description inview of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional VFD driven motor system.

FIG. 2A shows grounding of an inverter bridge across the transistormodule's insulating plate and resulting inverter bridge capacitance,according to a conventional VFD system.

FIG. 2B shows the series connection of the inverter bridge to groundacross the transistor module's insulating plate, the module's heat sinkplate, and the electrically insulating, high dielectric strength, lowdielectric constant, and high thermal conductivity plate according to anembodiment of the present invention.

FIG. 3A shows a schematic representation of a ground current loop in aconventional VFD system, as shown for example in FIG. 1, to include thecapacitance of the inverter bridge.

FIG. 3B shows a schematic representation of a VFD system having afloating neutral point on the DC bus in the absence of the presentinvention.

FIG. 4 shows a schematic representation of a ground current loop in aVFD system implementing an exemplary embodiment of the presentinvention, showing the capacitance of the transistor module and thecapacitance of the low dielectric constant insulating plate.

FIG. 5 shows another exemplary embodiment of the present inventioncomprising a common mode filter installed on the three phase drivecables.

FIG. 6 shows an exemplary embodiment of a mounting means which enableselectrical insulation and thermal conduction between the VFD powersemiconductor module and the grounded cooling plate and secures the VFDpower semiconductor module to the insulating plate and secures theinsulating plate to the grounded cooling plate.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention reduces ground leakage current by allowing theneutral point N of the DC bus to float without exposing the transistorsin the inverter bridge to excessive voltage spikes.

Turning first to FIG. 2A, the conventional grounding of an inverterbridge in a transistor module is shown. Grounding of the inverter bridgeis via the transistor module's heat sink plate 126. A capacitance C_(IB)62 is formed between the semiconductor substrate 122 and the heat sinkplate 126 across the thin insulating plate 124. The semiconductorsubstrate 122, the thin insulating plate 124, and the heat sink plate126 collectively form a conventional VFD power semiconductor module,also commonly referred to as a VFD transistor module. The heat sinkplate 126 is then mounted on and electrically connected to the groundedcooling plate 130.

FIG. 2B shows grounding of an inverter bridge in accordance with anexemplary embodiment of the present invention. The transistor module'ssemiconductor substrate 122, insulating plate 124 and heat sink plate126 are connected together as in the conventional grounding means ofFIG. 2A. However, according to an exemplary embodiment, an electricalinsulator plate P 175 having high dielectric strength, low dielectricconstant, and high thermal conductivity is mounted between the heat sinkplate 126 and the grounded cooling plate 130. The heat sink plate istypically made of aluminum silicon carbide. However, the heat sink plate126 need not be made of aluminum silicon carbide; a material providinggood electrical conduction and thermal conduction would be an adequatesubstitute.

The inverter bridge semiconductor substrate 122 is thermally connectedto and electrically insulated from the cooling plate 130 via plate P175. Because the dielectric constant of insulating plate P is low, asmall capacitance C_(P) 176 is formed between the transistor module'sheat sink plate 126 and the cooling plate 130. This capacitance issmaller than and is in series with the transistors internal capacitorC_(IB) 62 (FIGS. 2A, 2B, and 4). Capacitance C_(IB) 62 is a byproduct ofthe capacitive coupling between the semiconductor substrate surface 122and the transistor heat sink plate 126, which are insulated from eachother with the thin insulator 124, as shown in FIG. 2A.

Referring to FIG. 3A, in the conventional inverter bridge, grounding theneutral point N 26 of the DC bus 20 and grounding one end of C_(IB) 62to the same TE ground 80 creates a path to the DC Bus for displacementcurrents generated at every pulse transition. This grounding preventsone transistor from affecting another transistor in the inverter bridge.More particularly, removal of the neutral point N 26 grounding of the DCbus 20 causes one side of C_(IB) 62 to be floating relative to theneutral point N 26 of the DC bus, as shown in FIG. 3B, which allowsdisplacement current crosstalk between transistors in the inverterbridge via respective transistor capacitances, C_(IB) 62, which areinterconnected via cooling plate 130. This crosstalk can cause excessivevoltage spikes between the transistor module's semiconductor substrate122 and its heat sink plate 126 that can lead to component failure.

It is known by those skilled in the art that modular or isolated basehigh voltage IGBT transistors require the neutral point of the DC bus tobe grounded. Such grounding ensures that the maximum voltage between thetransistor terminals and the cooling plate is no more than half of themaximum DC bus voltage. If the neutral point is disconnected fromground, the capacitor, which is formed between the transistor module'ssemiconductor substrate and its heat sink plate, C_(IB) is stillgrounded on the cooling plate 130 side. The inverter bridge capacitanceis now, floating relative to the DC bus positive and negative voltages22/23 and is electrically connected to the other transistors' internalcapacitances, C_(IB) 62, in the inverter bridge via the grounded coolingplate 130. When the transistors which are attached to the positivevoltage of the DC bus are turned off, the semiconductor substrate isconnected directly to the positive voltage of the DC bus. Then, when thetransistors connected to the negative voltage are turned on, this causesinternal transistor capacitances, C_(IB) 62, to be charged at thenegative voltage potential of the DC bus. Because one side of all of thetransistors' respective internal capacitors, C_(IB) 62, are connectedtogether, the semiconductor substrate of the transistors connected tothe positive voltage are subjected to the full DC bus voltage. In thetypical case, the full DC bus voltage is substantially higher than thetransistor's insulation voltage rating and damage to the transistoroccurs.

FIG. 4 shows a schematic representation 304 of an exemplary embodimentof a VFD system according to the present invention incorporating aninsulating plate P 175, as shown for example in FIG. 2B. The insulatorplate capacitance C_(P) 176 is now in series with C_(IB) 62. Theinsulator plate capacitance, C_(P) 176 is connected to the TE groundedcooling plate 130. The series capacitance of C_(IB) and C_(P) provideshigher impedance for the return path to the voltage source V 140,decreasing ground leakage current I_(GND) 204. The insulating plate P175 (FIG. 2B) also serves to increase the insulation strength betweenthe cooling plate and the power semiconductor. Total system capacitanceC_(SYS) is decreased as described by equations 1 and 2 corresponding tothe circuit shown in FIG. 4. First, in equation 1, we define thecapacitance of the VFD, C_(VFD).

$\begin{matrix}{C_{VFD} = \frac{\left( {C_{P}*C_{IB}} \right)}{\left( {C_{P} + C_{IB}} \right)}} & (1) \\{C_{SYS} = \frac{\left( {\left( {C_{C} + C_{M}} \right)*C_{VFD}} \right)}{\left( {C_{C} + C_{M} + C_{VFD}} \right)}} & (2)\end{matrix}$

The decrease in total system capacitance C_(SYS) in turn reduces theground leakage current I_(GND) 204 from the voltage transitions at theoutput of the VFD according to equation 3.

$\begin{matrix}{I^{\prime} = {C_{SYS}*\left( \frac{V}{t} \right)}} & (3)\end{matrix}$

The following experimental data shown in Tables 1 and 2 was obtained inthe presence and absence of an exemplary embodiment of the presentinvention and confirms the effectiveness thereof. Table 1 summarizes thedata obtained under control conditions. The VFD module is a 4160 Voutput Microdrive (SMC Electrical Products, U.S. Pat. No. 6,822,866).The motor is a 500 HP induction motor rated at 4000 V or less. Groundcurrent measurements were made for three phase shielded cable lengths of30, 250, and 1300 feet. Ground current was continuously measured at theoutput terminals of the VFD and at the motor. The VFD was switching at 1kHz, and the motor speed was maintained at 30 percent. Controlmeasurements could not be made with the neutral point N of the DC busdisconnected from ground and floating in the absence of an insulatingplate P 175, as depicted in FIG. 3B. Disconnecting the neutral point Nfrom ground left the transistors of the inverter bridge only protectedby thin insulator 124, resulting in component failure due to crosstalk,as expected and as discussed above. A 1.5 μF capacitor was connected inseries from the neutral point N to ground and in parallel with C_(IB),to serve as a very high impedance path, providing more protection thanan open circuit. The 1.5 μF capacitor provides a high impedance path forthe neutral point to ground without sacrificing the inverter bridge.Table 1 summarizes the control data obtained without the insulatingplate P 175. Current values in Tables 1-3 are RMS.

TABLE 1 GROUND CURRENT MEASUREMENTS FOR 30, 250, and 1300 ft. CABLES(Amps) Without Boron Nitride Plate 30′ 250′ 1300′ @Drive 4.7 8.0 38.3@Motor 2.3 2.0 3.7

Table 2 summarizes experimental data obtained using an exemplaryembodiment of the present invention. The test conditions were the sameas those of the control conditions, above, with the following testmodifications. An insulating plate P 175 was installed between the heatsink plate 126 and the cooling plate 130 (as shown in FIGS. 2B and 6).The neutral point N of the DC bus was disconnected from ground andfloating. In this exemplary test embodiment, the insulating plate P isceramic made from boron nitride. FIG. 4 is a schematic representation ofthe test conditions summarized in Table 2.

TABLE 2 GROUND CURRENT MEASUREMENTS FOR 30, 250, and 1300 ft. CABLES(Amps) WITH Boron Nitride Plate 30′ 250′ 1300′ @Drive 0.7 1.1 5.5 @Motor0.5 0.3 0.4

Additional experimental measurements were made for the system accordingto another exemplary embodiment, comprising a common-mode filter (CMF150), shown for example in FIG. 5. The CMF 150, transformer and resistorballast, is installed across the three phase cables connecting the VFDoutput to the motor. Acquisition of the data below in Table 3 was madewith a 500 HP induction motor running at 50 percent of full speed. Allother test conditions were the same as those employed to acquire thetest data summarized in Table 2. The VFD was switching at frequency of 1kHz. Ground current measurements were made at the output terminals ofthe VFD and at the motor. Current values shown are RMS, as above.

TABLE 3 GROUND CURRENT MEASUREMENTS 250 ft CABLES (Amps) WITH BoronNitride Plate No CMF CMF @Drive 5.2 0.05 @Motor 0.8 0.05

As seen from the data in Table 3, above, ground leakage current isreduced to a negligible amount, 50 mA, with installation of the boronnitride ceramic plate and the CMF according to another exemplaryembodiment of the present invention.

While boron nitride plates were used in the exemplary embodiments forexperimental data acquisition described above, other oxide and nitridematerials or other insulating substances that have the desireddielectric and thermal properties described according to the presentinvention can be used. For example, synthetic diamond plates can be usedwhich have the desired dielectric properties and excellent thermalconduction.

Installation of the insulation plate 175, as shown in FIG. 2B, alsoprovides another object of the present invention, which is to enable theuse of transistors rated at less than half of the full DC bus potentialin building a reliable VFD system with the neutral point of the DC busgrounded. For example, for a motor rated at greater than 4160 V, a VFDproviding an output voltage greater than 4160 V, for example 6.9 kV, isdesirable. DC buses rated at 11.5 kV are readily available for use inVFDs and are adequate to provide a VFD output of 6.9 kV. Transistors tobuild the inverter bridge are readily available at a rating of 5,100V.Even when the neutral point is grounded, the transistor insulation(i.e., element 124 in FIG. 2A) breakdown voltage (5100 V) is less thanhalf the full potential on the DC bus 11.5 kV. Installation ofinsulating plate P 175, as shown in FIG. 2B, with sufficient dielectricstrength as described above, increases the effective insulation of theinverter bridge transistors analogous to the effect of C_(P) in serieswith C_(IB) versus C_(IB) alone. The increased insulation improvescomponent life and system reliability. Therefore, installation of theinsulation plate P 175 enables multiple transistors to be connected inseries (shown for example in FIGS. 1 and 5, S1-S12) and cooled by acommon cooling plate 130 (shown for example in FIG. 2B) to achieve ahigher output voltage VFD system at low cost using presently availabletransistors, rated at below half the full DC bus voltage, whilemaintaining the benefits of a single grounded cooling plate.

FIG. 6 shows an exemplary embodiment of a mounting means which enableselectrical insulation and thermal conduction between the VFD powersemiconductor module 50 and the grounded cooling plate 130 and securesthe power semiconductor to the insulating plate 175 and secures theinsulating plate to the grounded cooling plate 130. The VFD powersemiconductor module 50 (also shown in FIGS. 2A and 2B) is securelymounted on the insulating plate 175, which is securely mounted on thegrounded cooling plate 130 by means of L shaped steel brackets 180.Brackets 180 are fastened directly to the cooling plate 130 on one endand secure the VFD power semiconductor module via set screws 182 andceramic pellets 184 on the other end.

FIG. 6 shows only one exemplary embodiment of many possible means forsecuring the VFD power semiconductor module and the insulation plate tothe cooling plate. The bracket can be made from any material havingsufficient strength to support the clamping forces required by thetransistor module specifications. One ordinarily skilled in the art willreadily appreciate the various ways of physically securing the VFD powersemiconductor module to the insulating plate and the insulating plate tothe cooling plate while permitting the electrical insulation and thermalconduction capacities of the insulation plate to be realized.

In summary, reduction of ground leakage current in a VFD system isdesirable for multiple reasons in numerous application environments. Anelectrical insulator plate having high dielectric strength, lowdielectric constant, and high thermal conductivity mounted between theVFD power semiconductor module and the grounded cooling plate is aneffective means of reducing system capacitance thereby reducing groundleakage currents induced with the high frequency voltage shifts of aVFD. Installation of the insulator plate described above protects thetransistors in the inverter bridge from insulation breakdown byincreasing the insulation between the VFD power semiconductor module andground.

While the present invention has been particularly shown and describedaccording to exemplary embodiments herein, it will be understood bythose skilled in the art that various changes can be made in form ordetail without departing from the spirit and scope of the invention asdefined by the following claims.

1. A method of reducing ground leakage current in a VFD systemcomprising: floating a neutral point on a DC bus of the VFD, anddisposing an electrically insulating plate which has high thermalconductivity, a low dielectric constant, and high dielectric strengthbetween a VFD power semiconductor module and a cooling plate.
 2. Themethod of reducing ground leakage current in a VFD system according toclaim 1, further comprising: connecting a first end of cables torespective output terminals of the VFD and connecting a second end ofthe cables to a device to be driven by the VFD.
 3. The method ofreducing ground leakage current in a VFD system according to claim 2,further comprising: applying a common mode filter to the cablesconnected to the respective output terminals of the VFD.
 4. The methodof reducing ground leakage current in a VFD system according to claim 2,wherein the ground leakage current is reduced to less than 1.5 amps. 5.The method of reducing ground leakage current in a VFD system accordingto claim 3, wherein the ground leakage current is reduced to less than 1amp.
 6. The method of reducing ground leakage current in a VFD systemaccording to claim 4, wherein the ground leakage current is reduced toless than 1.5 amp, and wherein reduction to less than 1.5 amp isachieved despite a capacitance to ground due to the cables connected tothe output terminals of the VFD.
 7. The method of reducing groundleakage current in a VFD system according to claim 4, further comprisingdriving a motor using an output voltage of the VFD, wherein the groundleakage current is reduced to less than 1.5 amp, and wherein reductionto less than 1.5 amp is achieved despite a motor capacitance.
 8. Themethod of reducing ground leakage current in a VFD system according toclaim 5, wherein the ground leakage current is reduced to less than 1amp, and wherein reduction to less than 1 amp is achieved despite acapacitance to ground due to the cables connected to the outputterminals of the VFD.
 9. The method of reducing ground leakage currentin a VFD system according to claim 5, further comprising driving a motorusing an output voltage of the VFD and wherein the ground leakagecurrent is reduced to less than 1 amp, and wherein reduction to lessthan 1 amp is achieved despite a motor capacitance.
 10. A method ofreducing ground leakage current in a medium voltage VFD systemcomprising: floating a neutral point on a DC bus of the VFD; anddisposing an electrically insulating plate which has high thermalconductivity, a low dielectric constant, and high dielectric strengthbetween a VFD power semiconductor module and a cooling plate, whereinthe ground leakage current is reduced to less than 1.5 amps.
 11. Amedium voltage VFD system having low ground leakage current comprising:a DC bus which has a floating neutral point; an inverter bridge, of aVFD power semiconductor module, electrically connected to the DC bus; anelectrically insulating plate which has high thermal conductivity, a lowdielectric constant, and high dielectric strength; and a cooling plate,wherein the VFD power semiconductor module is mounted on theelectrically insulating plate, having high thermal conductivity, a lowdielectric constant, and high dielectric strength; wherein theinsulating plate is mounted on the cooling plate; and wherein thecooling plate is grounded.
 12. The medium voltage VFD system having lowground leakage current according to claim 11, further comprising: afirst end of cables connected to respective output terminals of the VFD,a common mode filter applied to the cables and filtering an output ofthe VFD, and a second end of the cables connected to a device to bedriven by the VFD.
 13. A medium voltage VFD system having low groundleakage current according to claim 12, wherein the ground leakagecurrent is reduced to less than 1 amp.
 14. A medium voltage VFD systemhaving low ground leakage current according to claim 11, wherein theground leakage current is reduced to less than 1.5 amps.
 15. A mediumvoltage VFD system having low ground leakage current according to claim13, wherein the cables connected to the output terminals of the VFD arein excess of 30 feet.
 16. A medium voltage VFD system having low groundleakage current according to claim 12, wherein an output voltage on theVFD output terminals is greater than 690 volts.
 17. The medium voltageVFD system having low ground leakage current according to claim 11,wherein the insulating plate is a ceramic material.
 18. The mediumvoltage VFD system having low ground leakage current according to claim11, wherein the insulating plate is made of boron nitride.
 19. Themedium voltage VFD system having low ground leakage current according toclaim 11, wherein a capacitance is formed across the electricallyinsulating plate between the VFD power semiconductor module and thecooling plate, and wherein said capacitance between the VFD powersemiconductor module and the cooling plate is less than an inverterbridge transistor internal capacitance to ground, and a totalcapacitance to ground of the VFD system is reduced.
 20. The mediumvoltage VFD system having low ground leakage current according to claim11, further comprising at least one mounting device which enableselectrical insulation and thermal conduction between the VFD powersemiconductor module and the grounded cooling plate and secures the VFDpower semiconductor to the insulating plate and secures the insulatingplate to the grounded cooling plate.
 21. A medium voltage VFD systemcomprising: a DC bus which has a grounded neutral point; VFD powersemiconductor modules, wherein transistor modules of an inverter bridgeare connected in series, the inverter bridges is electrically connectedto the DC bus, and an insulation of transistor modules is rated at lessthan half of a full maximum DC bus voltage rating; an electricallyinsulating plate which has high thermal conductivity, a low dielectricconstant, and high dielectric strength; and a grounded cooling plate,wherein the VFD power semiconductor module is mounted on theelectrically insulating plate, having high thermal conductivity, a lowdielectric constant, and high dielectric strength; wherein theinsulating plate is mounted on the grounded cooling plate; and whereinan effective insulation capability to ground of the VFD system isincreased.